The present invention relates to a method for addressing a flat screen, more particularly a liquid-crystal display screen, using pixel precharging. The present invention also relates to a column driver of such a screen, for implementing the method, as well as the application of the method to large screens.
Direct-view or projection liquid-crystal display screens are generally composed of lines (selection lines) and columns (data lines), with the pixel electrodes, connected through transistors to these lines, being located at their intersections. The gates of these transistors form the selection lines and are driven by the peripheral drivers which scan the lines and turn on the transistors of each line, to make it possible, by means of the data lines connected to the other peripheral drivers, to charge the pixel electrodes and modify the optical properties of the liquid crystal contained between these electrodes and the backing electrode (or reference electrode), thus making it possible to form images on the screen.
FIG. 1 represents the equivalent circuit diagram of a flat-screen pixel addressed by the line and column drivers. The electrode and the backing electrode enclosing the liquid crystal form a capacitor 1 whose charge (most often consisting of video data) is transmitted by the column 2 through the transistor 3 driven by the selection line 4. For its part, FIG. 2 represents the time profiles of the operation of this pixel, Vs being the signal addressed by the selection line of a row of pixels, Vc being the video signal sampled from the selected row of pixels and Vp being the effective charge of one of these pixels. In theory, at the end of a sampling pulse, the pixel voltage Vp across the terminals of the liquid crystal should be equal to the column voltage Vc, that is to say +/xe2x88x92V.
The problem with this type of addressing is that, in practice, the voltage Vp is different from the charging voltage Vc of the column. This is because, when it is on, each transistor 3 has a non-zero resistance Ron, so that the charge of the pixel exhibits an exponential characteristic (as represented in FIG. 2) whose time constant is non-zero since it is equal to the product Ronxc3x97C, C being the capacitance of the pixel capacitor 1. When the charging time has elapsed, the residual convergence error is equal to Ven+ in positive frame (negative value) or Venxe2x88x92 in negative frame (positive value), which are different from the values +/xe2x88x92V of the charging voltage Vc.
This results in an error on the RMS voltage tilting the liquid crystal of the order of (Ven+xe2x88x92Ven)/2. However, the electro-optical specifications of a screen set a maximum value for this error, of the order of 5 to 10 mV for a 90xc2x0 twisted nematic effect. The product RC (resistance times capacitance) must therefore typically be 7 to 8 times less than the addressing time in order to achieve a convergence rate which is compatible with a high-quality application. This entails limitations on the number of lines which can be addressed as well as on the size of the pixels. In this case, R needs to be reduced, that is to say the transistor needs to be widened. This is not realistic beyond a channel width-to-length ratio of more than a few units. Furthermore, when the pulse Vs applied to the selection line returns to the low state (see FIG. 2), the parasitic coupling between the line and the pixel becomes excessive when the transistor width exceeds a certain value.
Another known solution is represented in FIG. 3. In this case, a screen 5 consisting of pixels 6 is addressed by a line driver 7 and a column driver 8 which is formed by samplers driven by a shift register. The load of a sampler is none other than the distributed capacitance of the driven column 9. This column needs to be charged over a very short time, with the above-mentioned conversion problems aggravated by the fact that the charging time is no more than a fraction of the time when a line 9 is addressed. This is because, during this line time, the video needs to be sampled successively over all the columns of the screen. For this reason, the production of integrated-driver screens has to date required the use of a high-mobility semiconductor, for example monocrystalline or polycrystalline silicon.
In order to overcome the above drawbacks, and to allow the use of thin-film transistors produced in silicon, it has been proposed, in particular in application PCT/FR94/16428, to precharge the pixels to a voltage lower than the working voltage. There are a number of drawbacks with using a voltage of this type. In particular, it does not solve the convergence problem.
The present invention provides a novel addressing method for overcoming the drawbacks mentioned above.
The present invention accordingly relates to a method for addressing a flat screen composed of lines and columns, with pixels located at their intersections, characterized in that, at the start of each sampling of the video signal to be displayed on the screen, a voltage (Vr) higher than the working voltage range (V) is applied to the selected pixel for a time tr, then the working voltage is sampled for a time ts.
Preferably, the precharge voltage (Vr) is chosen such that Ven+=Venxe2x88x92 where Ven+ and Venxe2x88x92 represent the residual error respectively in positive frame and in negative frame. In this case, the precharge voltage is obtained by the following formula:       Ven    +=                            (                      Vr            -            V            +                    )                ⁢        exp            ⁢              xe2x80x83            -                        ts                      τ            ⁢                          xe2x80x83                        ⁢                          (                              Vg                -                Vt                -                V                +                            )                                      ⁢                  xe2x80x83                ⁢        and                        xe2x80x83        ⁢          Ven      -=                                    (                          Vr              -              V              -                        )                    ⁢          exp                -                  ts                      τ            ⁢                          xe2x80x83                        ⁢                          (                              Vg                -                Vt                -                V                -                            )                                            ⁢          xe2x80x83      
Where Vg is the gate voltage of the transistor during the sampling and Vt is its threshold voltage.
The condition Ven+=Venxe2x88x92 is written:       (          Vr      -      Vt        )    =                    (                  Vr          -          V          -                )            ⁢      exp        -          ts      ⁢              (                              1                          τ              ⁢                              xe2x80x83                            ⁢                              (                                  Vg                  -                  Vt                  -                  V                  -                                )                                              -                      1                          τ              ⁢                              xe2x80x83                            ⁢                              (                                  Vg                  -                  Vt                  -                  V                  +                                )                                                    )            
or xcfx84(Vgxe2x88x92Vtxe2x88x92Vxe2x88x92)=Ron(Vgxe2x88x92Vtxe2x88x92Vxe2x88x92)xc3x97C and   Ron  =      1          μ      ⁢              xe2x80x83            ⁢      Cox      ⁢              xe2x80x83            ⁢              W        L            ⁢              (                  Vg          -          Vt          -          V          -                )            
whence xcfx84(V) is of the form   Cte  V
whence       (          Vr      -      V      +        )    =                    (                  Vr          -          V          -                )            ⁢      exp        -          ts              τ        ⁢                  xe2x80x83                ⁢                  (                      V            +                          -              V                        -                    )                    
i.e.   Vr  =      V    +                  (                  V          +                      -            V                    -                )            ⁢              xe2x80x83            ⁢                        exp          -                      xe2x80x83                    ⁢                      ts                          τ              ⁢                              xe2x80x83                            ⁢                              (                                  V                  +                                      -                    V                                    -                                )                                                              1          -          exp          -                      ts                          τ              ⁢                              xe2x80x83                            ⁢                              (                                  V                  +                                      -                    V                                    -                                )                                                        
The present invention also relates to a column driver of a flat screen of the type comprising samplers driven by the outputs of the shift register, characterized in that each sampler consists of three Metal-Insulator-Semiconductor (MIS)-type transistors mounted in parallel so that their first electrode is connected to the video signal and their second electrode is connected to the driven column, the gate of the first transistor being connected to one of the outputs of the shift register and the gates of the second and third transistors being connected to two clocks chosen so that one of the two transistors is activated to precharge the even frames and the other is activated to precharge the odd frames.
According to another characteristic of the invention, the clock voltage applied to the second and third transistors is chosen so that, when a transistor is not being used for the precharging, its gate receives a negative voltage allowing subsequent compensation for the capacitive coupling when this voltage returns to zero.
Preferably, the three transistors are identical and are thin-film transistors, TFTs. This solution makes it possible to compensate for the strong capacitive coupling, because the transistors used to produce the samplers are large. It furthermore makes it possible to distribute the stress or fatigue evenly over the three transistors, which have the same size, this having the effect of increasing the life of the transistors.
The present invention also relates to the application of the above addressing method to large screens.
The present invention therefore relates to a method for addressing a flat screen including lines and columns, with pixels located at their intersections, in which X line drivers are each connected to Y lines, characterized in that, for a time tr, the pixels located on the lines connected to the first line driver are precharged to a voltage (Vr) higher than the working voltage range (V), then the Y lines are sampled successively and the above operation is repeated for the X-1 remaining drivers
The present invention also relates to a method for addressing a flat screen including lines and columns, with pixels located at their intersections, in which X line drivers are each connected to Y lines, characterized in that the first line of each of the X line drivers is simultaneously precharged to a voltage Vr higher than the working voltage range (V) and the said line of the X line drivers is then sampled successively and the above operation is repeated for the Y-1 other lines of each of the X line drivers.
The present invention will be understood more clearly, and additional advantages will emerge, on reading the following description which is illustrated by the following figures:
FIG. 1, already described, represents the equivalent circuit diagram of a pixel of a liquid-crystal display screen,
FIG. 2, already described, represents the time diagrams of the operation of the pixel in FIG. 1,
FIG. 3, already described, represents a known structure of a screen driven by line and column drivers,
FIG. 4 illustrates a method of addressing a liquid-crystal display screen according to the present invention,
FIG. 5 represents one embodiment of a known column driver employing the addressing method according to the present invention,
FIG. 6 represents the time diagram of a column driver according to FIG. 5,
FIG. 7 represents a preferred embodiment of a column driver employing the method according to the present invention,
FIG. 8 represents the time diagram of the operation of the column driver in FIG. 7, and
FIG. 9 schematically represents a part of a large flat screen connected to line and column drivers using the method of the present invention.